1. Field of the Invention
The present invention relates to an organic electrolyte battery which is provided with an internal safety switch to protect the battery from overcurrent, which lead to the damage or the disruption of the battery seal and the anomalous temperature rise.
2. Description of the Prior Art
One of the high density energy batteries now in use is the organic electrolyte battery in which the anode active material is metallic lithium or a carbonaceous material (such as coke) capable of doping and dedoping lithium and the electrolyte is an organic solvent. Unfortunately, it suffers a disadvantage of heating up (higher than 200.degree. C.) due to overcurrent resulting from forced discharge or external short-circuit. The consequence of heating up is the evaporation or decomposition of the organic electrolyte (organic solvent) which breaks or bursts the battery.
For protection of the organic electrolyte battery from overcurrent and heating up, there have been proposed several kinds of internal safety switches. For example, Japanese Patent Laid-open No. 191273/1984 discloses a battery provided with a terminal conductor made of a shape memory alloy which cuts off the battery current when the battery temperature rises. U.S. Pat. No. 4,035,552 discloses a battery provided with two metal plates of different materials between the battery cover and casing which plates function as a switch responding to heat and pressure. There is another known way of controlling and preventing overcurrent by means of a PTC resin resistor which rapidly increases in resistance, thereby cutting off current, when the temperature reaches a certain threshold value. (PTC stands for positive temperature coefficient.)
In practice, the PTC resistor is interposed between the electrode (the battery casing seal) and the terminal cap, as disclosed in Japanese Utility Model Laid-open No. 3476/1984. An organic electrolyte battery provided with a PTC resistor is shown in section in FIG. 3. It is a secondary battery of cylindrical shape made up of an electrode assembly, an electrolyte, and a battery can 4 containing them. The electrode assembly consists of a long cathode 1 (having a lithium compound as the active material) and a long anode 2 (having a carbonaceous material as the active material) which are wound with a separator 3 interposed between them. The cathode 1 is connected to a sealing member 6 through a lead wire 5. Above the sealing member 6 is placed a vent-holed battery lid 8, with a PTC resistor 7 interposed between the lid 8 and member 6. The anode 2 is connected to the battery can 4. The battery is sealed by crimping the open end of the battery can 4 in such a manner as to enclose the edges of the sealing member 6, PTC resistor 7, and battery lid 8, with an insulating gasket 9 interposed between them.
The battery constructed as mentioned above controls itself according to its temperature. When the temperature reaches a certain threshold value, the PTC resistor 7 rapidly increases in resistance, thereby reducing the flow of current in the battery. This protects the battery from overheating (and its resulting pressure increase) due to overcurrent and, hence, prevents the leakage and bursting of the battery. When the temperature goes down below the threshold value, the PTC resistor 7 decreases in resistance accordingly, restoring the battery to its normal state. Incidentally, to ensure the safety of the battery in case the PTC resistor 7 does not work as intended, the battery has a safety valve in the sealing member or sealer 6 which opens to break the lead wire 5 and permits gas to discharge through the vent hole in the battery lid when the temperature in the battery anomalously increases.
A disadvantage of the conventional organic electrolyte battery provided with a PTC resistor to control and prevent overcurrent is that the open end of the battery can 4 cannot be crimped tightly because the mechanically weak PTC resistor is broken by tight crimping. Loose crimping is liable to cause the leakage of electrolyte from the seal when the seal is deformed by a temperature rise. Moreover, the PTC and battery lid are subject to corrosion at their contact, which is unfavorable to battery storage over a long period of time.